User:Satishb.elec/sandbox

CAPACITANCES RELATED TO PN-JUNCTION DIODE

1) Transition capacitance

   The decrease in uncovered charges with applied voltage may be considered as a capacitive effect,which may be reffered as transition capacitance or space charge capacitance.
   It can be reprented as  
                     Ct=|dQ/dV|  (which shows Ct is not a constant quantity.)
  This Ct is not constant but depends on the magnitude of reverse voltage.the value of transition capacitance can be given by the formula, 
    Ct=E*a/Wp
  Larger the reverse bias voltage,larger is the space charge width and smaller the capacitance Ct.Similarly,for increase in forward bias voltage bandwidth will decrease and W decreases and Ct will increase.
In case of forward bias it's value is very low and hense not considered.

Examples of transition capacitor:

            *Varactor Diode- We have the diode named as varactor diode which can be used for certain purposes which has the property of space charge capacitance.Larger the reverse voltage,larger the space charge width and smaller the capacitance Ct.And similarly larger the forward voltage smaller the width and larger the Ct. It can be used in many electrical circuits like tunning of LC resonant circuit,Self biasing bridge circuit and in special types of amplifiers called parametric amplifires.

2) Diffusion Capacitance

   In case of forward biased diode this capacitance play more role than that of space charge capacitance.this may be defined as rate of change of injected chargewith applide voltage.
   Cd=(Tp*I)/(n*Vt)
               where, Cd-diffusion capacitance  
                      Tp-life time of minority charge carriers (here holes)
                      n-diode material constant(2-for Si,1-for Ge) 
                      Vt-thermal vvoltage.